66 Dart electronic ignition issues
Either
bad connection at ballast resistor
dark blue "run" voltage is not getting to ballast
bad ballast resistor
Turn the key to "run" with engine stopped, put your meter / light and check both sides of the ballast resistor. If you have battery voltage on one side but not the other, check resistor and it's connections
If you have NO voltage on either terminal, check the bulkhead connector for bad connections, or any wiring splices that may have been made/ altered
Also check and see if the IGN terminal of the regulator has voltage, and the blue (switched ignition) field wire of the alternator has power
ALL THREE should all have power with key in "run"
If one or more does, but the ignition does not, the trouble is on the engine side of the bulkhead and in the harness -- some splice somewhere
If you DO NOT have voltage at any of those three points, check right where the dark blue IGN comes out of the bulkhead. If none there, the trouble is in the bulkhead connection, or under the dash, ignition switch, switch connector.
Do the warning lights / gauges work with key in "run?"